Power MOSFETs (metal-oxide-semiconductor field-effect transistors) comprise one of the most useful field effect transistors implemented in both analog and digital circuit applications.
In general, a trench-based power MOSFET is built using a vertical structure as opposed to a planar structure. The vertical structure enables the transistor to sustain both high blocking voltage and high current. Similarly, with a vertical structure, the component area is roughly proportional to the current it can sustain, and the component thickness is proportional to the breakdown voltage.
The geometry of the features of the power MOSFET components are commonly defined photographically through photolithography. The photolithography process is used to define component regions and build up components one layer on top of another. Complex devices can often have many different built up layers, each layer having components, each layer having differing interconnections, and each layer stacked on top of the previous layer. The resulting topography of these complex devices often resemble familiar terrestrial “mountain ranges”, with many “hills” and “valleys” as the device components are built up on the underlying surface of the silicon wafer.
There exists a problem, however, in the fact that prior art power MOSFET components have active areas with a considerable amount of topography. The active areas of the prior art power MOSFET devices have many hills and valleys from the layered components built up on the underlying silicon. To enable component interconnects, this topography is covered by a thick metalization layer that is optimized to fill in the valleys and cover the peaks. This metalization layer is typically more than several microns thick (e.g., in common high-density power MOSFET devices).
The thick metalization layer leads to number problems. One problem is the fact that even though the metalization layer is designed to fill in the valleys, there can exist voids where the valleys are too narrow to allow effective filling. Such voids become prime areas for the introduction of flaws into the completed power MOSFET device. Another problem is the fact that the deposition of such a thick metalization layer is a very expensive step in the fabrication process. Accordingly, what is needed is a power MOSFET fabrication process that avoids the thick metalization layer problems on planarized topography services.